In producing hydrocarbons or the like from loosely or unconsolidated and/or fractured formations, it is not uncommon to produce large volumes of particulate material along with the formation fluids. As is well-known in the art, these particulates routinely cause a variety of problems and must be controlled in order for production to be economical. Probably, the most popular technique used for controlling the production of particulates (e.g., sand) from a well is one which is commonly known as "gravel-packing."
In a typical gravel-packed completion, a screen is lowered into the wellbore on a work string and is positioned adjacent to the subterranean formation to be completed, e.g., a production formation. Particulate material, collectively referred to as "gravel," and a carrier fluid is then pumped as a slurry down the work string where it exits through a "cross-over" into the well annulus formed between the screen and the well casing or open hole, as the case may be. The carder liquid in the slurry normally flows into the formation and/or through the screen itself, which, in turn, is sized to prevent flow of gravel therethrough. This results in the gravel being deposited or "screened out" in the well annulus where it collects to form a gravel pack around the screen. The gravel, in turn, is sized so that it forms a permeable mass which allows the flow of the produced fluids therethrough and into the screen while blocking the flow of the particulates produced with the production fluids.
One major problem that occurs in gravel-packing single zones, particularly where they are long or inclined, arises from the difficulty in distributing the gravel over the entire completion interval, i.e., completely packing the entire length of the well annulus around the screen. This poor distribution of gravel (i.e., incomplete packing of the interval) is often caused by the carrier fluid in the gravel slurry being lost into the more permeable portions of the formation which, in turn, causes the gravel to form "sand bridges" in the annulus before all the gravel has been placed. Such bridges block further flow of slurry through the annulus which prevents the placement of sufficient gravel (a) below the bridge in top-to-bottom packing operations or (b) above the bridge in bottom-to-top packing operations.
To address this specific problem, "alternate path" well strings have been developed which provide for distribution of gravel throughout the entire completion interval, even if sand bridges form before all the gravel has been placed. Some examples of such screens include U.S. Pat. Nos. 4,945,991; 5,082,052; 5,113,935; 5,417,284; 5,419,394; 5,476,143; 5,341,880; and 5,515,915. In these well screens, the alternate paths (e.g., perforated shunts or bypass conduits) extend along the length of the screen and are in fluid communication with the gravel slurry as the slurry enters the well annulus around the screen. If a sand bridge forms in the annulus, the slurry is still free to flow through the conduits and out into the annulus through the perforations in the conduits to complete the filling of the annulus above and/or below the sand bridge.
One of the problems with the alternate path design is the relatively small size of the passages through them. These tubes are also subject to being crimped or otherwise damaged during the installation of the screen. Thus, several designs in the past have placed these tubes inside the outer surface of the screen. This type of design substantially increases the cost of the screen over commercially available screens. Yet other designs have recognized that it is more economical to place such tubes on the outsides of the screen and have attempted to put yet another shroud over the alternate paths which are on the outside of the screen to prevent them from being damaged during insertion or removal. Such a design is revealed in U.K application No. GB 2317 630 A.
While such designs can be of some benefit in a bridging situation, they present difficulties in attempting to treat and gravel-pack zones which are fairly close together. Many times zones are so close together that traditional isolation devices between the zones cannot be practically employed because the spacing is too short. For example, situations occur where an upper and lower zone are spaced only 5-20 feet from each other, thus precluding a complete completion assembly in between screens for each of the zones. When these closely spaced zones are encountered, it is desirable to be able to gravel-pack and treat the formations at the same time so as to save rig time by eliminating numerous trips into the well. At times these types of completions will also require some degree of isolation between them, while at the same time producing one or the other of the formations. Accordingly, the objective of the apparatus and method of the present invention is to facilitate fluid treatments such as fracture stimulation, as well as gravel-packing, simultaneously, in two or more adjacent producing zones. Another object of the method and apparatus of the present invention is to provide limited hydraulic isolation between two or more adjacent zones. Yet another object of the present invention is to minimize rig time for the completion by reducing the number of trips required to install the gravel screen assemblies and to treat the formation. These objectives and how they are accomplished will become more clear to those skilled in the art from a review of the detailed description of the preferred embodiment below.